TeamSpeak Server is the component that allows other clients to connect to a single place, so they can talk easy and secure. The Server comes as a package and can be installed on multiple platforms: Windows, Linux, Mac, Free BSD, etc. The server is different because it contains all the modules that make the voice chat possible. The one who installs the TeamSpeak Server is also the administrator of the server. Compared to the Client version, the Server is much more advanced, since it allows configuration of all aspects to the base level.
Download page: http://www.teamspeak.com/downloads#server
After it's set up, the server permits connection for other users that have Client installed.
You can read more on the following page: http://www.teamspeak.com/teamspeak3